As the hardware technique for image processing is being developed and also the user needs on image photographing is increasing, an auto focus (AF) function and an optical image stabilization (OIS) function are implemented not only at an independent camera but also at a camera module mounted to a mobile terminal such as a cellular phone and a smart phone.
The auto focus function is to adjust a focus distance to a subject by linearly using a lens or a lens assembly in an optical axis direction so that a clear image is generated at an image sensor (CMOS, CCD or the like) provided at a rear end of the lens.
In order to implement the auto focus function, various methods may be used. Representatively, a magnet (a permanent magnet) is installed at an AF carrier (or, a mover), a coil is installed at a stator (or, a housing, or another type of carrier), and a power of a suitable level is applied to the coil to generate an electromagnetic force to the coil (provided at the stator) and the magnet (provided at the mover) to move the mover in an optical axis direction.
In addition, recently, a device, or an actuator, in which the AF and OIS functions are integrated is used. In this case, a structure for moving an OIS carrier (or, a frame, a lens assembly or the like) having a lens loaded thereon in the AF carrier in an X-axis and/or Y-axis direction perpendicular to the optical axis is integrally implemented together with the AF structure described above. In some cases, a lens may be loaded on the AF carrier, and an OIS carrier provided out of the AF carrier may be provided to move in a direction perpendicular to the optical axis direction.
Meanwhile, in an existing device in which only the AF function is provided or the AF and OIS functions are provided together, balls 510-1, 510-2 arranged in the same direction as the optical axis are interposed between an AF carrier 500 (the mover) and a housing 400 (the stator, FIG. 2) in order to improve the behavior characteristics of the AF carrier 500 moving in the optical axis direction, as shown in FIGS. 1 and 2.
In this structure, a suitable distance may be continuously maintained between the mover and the stator, and a frictional force is minimized by means of rotation of the balls and point contacts with the balls, so that the AF carrier may move more smoothly and accurately in the optical axis direction.
In this case, a yoke 420 (FIG. 2) is mounted to the housing 400 (the stator, FIG. 2) to generate an attractive force to a magnet 520 provided at the AF carrier 500 so that the point contact between the AF carrier 500 and the ball 510 may be effectively maintained.
However, in the existing technique, in order to enhance the attractive force between the magnet 520 and the yoke, as shown in FIG. 1 and FIG. 2(a), the magnet 520 generating an attractive force with the yoke is designed as large as possible so that its height D2 is equal to or higher than an entire height D1 of the balls 510 which physically support the AF carrier 500 and the housing.
In this configuration, as shown in FIG. 2(b), if the AF carrier 500 moves downwards based on the optical axis direction by means of AF operation, the magnet 520 loaded on the AF carrier 500 also moves downwards, and thus an attractive force is relatively strongly applied between the magnet 520 and the yoke 420 in a region where the balls do not support, thereby generating a tilt fault θ1 in a horizontal direction.
In another point of view, FIG. 2(c) shows that the AF carrier 500 moves upwards on the basis of the optical axis direction by means of AF operation. In this case, a region where the balls do not support is generated above the magnet 520, and in this region, a detachment may be generated by a predetermined angle θ2 in a lower direction due to the attractive force between the yoke 420 and the magnet 520, thereby causing a tilt fault.
Therefore, in the existing technique, if the AF carrier 500 moves in the optical axis direction, an attractive force is applied between the magnet 520 and the yoke 420 in a region where the balls 510 do not physically support, which may break the balance of the AF carrier 500 and thus resultantly cause tilt faults θ1 and θ2 of the AF carrier 500 as shown in FIG. 2(d).
The tilt faults deform a light path passing to an image sensor 600 through the lens as much as a maximum separation angle (θ=θ1+θ2), thereby causing an error in focusing as much, and thus a problem is caused in generating a clear image.
Recently, a camera module loaded on a smart phone or the like is implemented with a lightweight and slim design. If the camera module has such a slim design, a ratio of width to thickness of the AF carrier increases further, and thus the tilt problem of the AF carrier may become more serious.